The present invention relates to a radio base station apparatus and loopback test method and, more particularly, to a radio base station apparatus and its loopback test method which perform a loopback test in which a signal to be exchanged between radio terminals is looped back inside the apparatus.
A radio base station apparatus used in mobile communication such as WCDMA (Wideband Code Division Multiple Access) has a loopback test function of testing the transmission/reception function for a signal to be exchanged between radio terminals. When, for example, the transmission function for a downstream signal to be transmitted from the radio base station apparatus to a radio terminal is to be tested, the downstream signal output from the transmission function is looped back inside the radio base station apparatus, and the looped-back signal is received by the reception function for an upstream signal to be transmitted from the radio terminal to the radio base station apparatus. The normality of the transmission function is then checked on the basis of the reception result. In contrast, when the reception function is to be tested, an upstream signal is output from the transmission function of transmitting a downstream signal, and the looped-back signal obtaining by looping back the downstream signal inside the radio base station apparatus is received by the reception function. The normality of the transmission function is then checked on the basis of the reception result.
In such a loopback test, a looped-back signal which is looped back inside the radio base station apparatus sometimes interferes with an upstream signal actually sent from a radio terminal in the reception function. The interference between the upstream signal and the looped-back signal needs, therefore, to be reduced by decreasing the level of the looped-back signal.
Conventionally, as a radio base station apparatus test method using a test transmitter and receiver (to be referred to as a TTR hereinafter), a technique for reducing such test signal interference has been proposed (see, for example, patent reference 1). In a test using a TTR, a predetermined test signal transmitted from the TTR is received by the radio base station apparatus, and the normality of the reception function of the radio base station apparatus is checked. In contrast, a predetermined test signal returned from the radio base station apparatus is received by the TTR, and the normality of the transmission function of the radio base station apparatus is checked.
In a test using such a TTR, a test signal with a relatively high level which is transmitted from the TTR interferes with an upstream signal from another radio terminal, or a test signal with a relatively high level which is transmitted from the radio base station apparatus interferes with a downstream signal to another radio terminal. For this reason, the lowest bit rate which is used as that of an upstream signal or downstream signal or a lower bit rate is used as that of a test signal transmitted from the TTR or radio base station apparatus. Since a test signal with a low bit rate increases in spreading gain, the transmission quality, i.e., the ratio (Eb/No) of a signal reception energy Eb per bit to a power spectrum density No of the sum of noise and despread interference noise can be improved. Therefore, when the radio base station apparatus or TTR transmits a test signal, the transmission power of the test signal can be reduced by the improvement in Eb/No. This makes it possible to reduce interference with another upstream signal or downstream signal.
In a mobile radio communication system like that based on the WCDMA scheme, in which a plurality of radio terminals are simultaneously call-connected, and the number of radio terminals which can be connected varies depending on the amount of interference, as the amount of interference increases, the number of terminals which can be connected to the radio base station apparatus decreases. For this reason, the transmission power of a TST signal must be set to a low value that has no influence on a general call. In addition, as the number of general calls increases, the quality of a TST signal deteriorates due to the interference between the calls, resulting in errors. TST is for checking normality in a radio base station apparatus, and hence a system designed to cause an error only when apparatus abnormality occurs is preferable. That is, signal deterioration due to such general call interference must be avoided.
In the above conventional technique, since TST transmission power is permanently determined, interference occurs between a TST signal used for a loopback test and a general call. This has influence on the general call and decreases the accuracy of TST. When, for example, TST transmission power is fixed, the SIR (Signal to Interference Ratio: the ratio between a desired signal and an interference noise sum) of a general call and TST is given as follows. Assume that the numbers of call connections are 1 and 64. In this case, if TST signal power Ctst (1)=Ctst (64)=−138 dBm, SIRtst (the ratio between a test signal and an interference noise sum) is given by
                    SIRtst        =                ⁢                              Ctst            ⁡                          (              64              )                                +                      10            ×            log            ⁢                                                  ⁢                          (              SF              )                                -                      10            ×            log            ⁢                                                  ⁢                          (                                                                    10                    NO                                    /                  10                                +                                                                                    ⁢                                            10                              C                ⁢                                                                  ⁢                1                ⁢                                  (                  64                  )                                                      /            10                    +                                    10                              C                ⁢                                                                  ⁢                2                ⁢                                  (                  64                  )                                                      /            10                    +          …          +                      10                          C              ⁢                                                          ⁢              64              ⁢                              (                64                )                                                    )                                =                ⁢                              -            12                    ⁢                                          ⁢          dB                    The quality represented by this equation is far from being accepted as that for passing a signal.
Assume that call connection count=1, C1(1)=−138 dBm, and Ctst(1)=Ctst(64)=−120 dBm. In this case, SIR1 and SIRtst are given by
                                          SIR            ⁢                                                  ⁢            1                    =                                    C              ⁢                                                          ⁢              1              ⁢                              (                1                )                                      +                          10              ×              log              ⁢                                                          ⁢                              (                SF                )                                      -                          10              ×              log              ⁢                                                          ⁢                              (                                                                            10                      NO                                        /                    10                                    +                                                            10                                              Ctst                        ⁡                                                  (                          1                          )                                                                                      /                    10                                                  )                                                                                  =                      3            ⁢                                                  ⁢            dB                                                            SIRtst          =                    ⁢                                    Ctst              ⁡                              (                1                )                                      +                          10              ×              log              ⁢                                                          ⁢                              (                SF                )                                      -                          10              ×              log              ⁢                                                          ⁢                              (                                                                            10                      NO                                        /                    10                                    +                                                            10                                              C                        ⁢                                                                                                  ⁢                        1                        ⁢                                                  (                          1                          )                                                                                      /                    10                                                  )                                                                                  =                    ⁢                      24            ⁢                                                  ⁢            dB                              In this case, since the target SIR value, i.e., TargetSIR=6 dB, the transmission power of the terminal is controlled to be increased by an amount corresponding to the degree to which SIR1 (the ratio between the terminal transmission signal and the interference noise sum) is less than 6 dB. In contrast to this, the TST signal quality becomes higher than necessary. In a loopback test, therefore, as the number of general calls increases, interference with loopback signals increases. As a result, an error occurs in a loopback signal, and the normality of the apparatus cannot be properly changed.